1. Field of the Present Invention
The present invention relates to a cell search method suitable for a code division multiple access (CDMA) system. More particularly, the present invention provides a cell search method suitable for a wideband CDMA (W-CDMA) system that can reduce the clock offset effects in the system.
2. Description of the Related Art
The demand for efficient transmissions of large amounts of data in ground mobile communication systems has facilitated the application of direct sequence spread spectrum (DSSS) transmissions. Code division multiple access (CDMA) technology cellular systems use DSSS. A DSSS system's channel capacity is substantially larger than the channel capaciw of a frequency-hopping spread spectrum system. Conventionally, due to frequency reuse, the bandwidth efficiency of a CDMA system is greater than that of other multiple access systems such as a frequency division multiple access (FMA) system or a time division multiple access (TDMA) system.
Furthermore, cell planning is easily achieved in the CDMA system. Therefore, it is foreseen that a CDMA cellular system will undergo substantial developments in the future. W-CDMA/FDD (frequency division duplex) systems, as discussed in the Third Generation Partnership Project (3GPP), has been already adopted as one of the standards for the International Mobile Telecommunication 2000 (IMT-200) third generation system.
In the CDMA cellular system the mobile station or user equipment has to initially find and synchronize itself to a base station in a process commonly called “cell search”. Fast cell search is reduces the on-switch delay of the mobile station (initial cell search), increases the standby time (idle mode cell search), and maintains connection quality during the handover or handoff operation (active mode cell search).
The U.S. Pat. No. 6,038,250, issued to Shou et al. and entitled “Initial synchronization method and receiver for DS-CDMA inter base station asynchronous cellular system”, the disclosure of which is incorporated by reference herein, proposes the implementation of an initial synchronization method in a high speed cell search and the utilization of a specific receiver in a DS-CDMA inter base station asynchronous cellular system. According to Shou et al., a base band received signal is delivered to a matched filter and is correlated with a spread code that is produced via a spread code generator. A signal electric power calculator determines the electric power of the output signal of the matched filter, and consequently delivers the result respectively to a long code synchronization timing determiner, to a threshold value calculator, and to a long code identifier. During the initial cell search, the spread code generator outputs a short code that is common to the control channel of each of the base stations. After the long code synchronization timing has been determined, each chip, which forms a portion of synthesized spread code sequence of N chips, is successively handed over and outputted.
In the U.S. Pat. No. 6,185,244, issued to Nystrom and entitled “Cell searching in a CDMA communication system”, the disclosure of which is incorporated herein by reference, a special coding scheme is proposed to more effectively acquire a long code and a frame timing during the cell search. As stated by Nystrom:
“A code set of M Q-ary code words length, including symbols from a set of Q short codes, is defined with certain properties. The primary property that has to be satisfied is that no cyclic shift of a code word yields a valid code word. The other properties to be satisfied are that there is one-to-one mapping between a long code message and a valid code word, and a decoder should be able to find both the random shift (thereby implicitly finding the frame timing) and the transmitted code word (i.e. its associated long code indication message) in the presence of interference and noise, with some degree of accuracy and reasonable complexity.”
In the U.S. Pat. No. 6,289,007, issued to Kim et al. and entitled “Method for acquiring a cell site station in asynchronous CDMA cellular communication systems”, the disclosure of which is incorporated herein by reference, a group code and a cell code, used as pilot codes, are multiplexed and used as a pilot code for discriminating a base station in an asynchronous cellular CDMA communication system. The pilot codes are used in different base stations of the asynchronous cell CDMA system. Multiplexing the pilot codes reduces interference. As disclosed by Kim et al., the proposed method acquires a cell site station in an asynchronous CDMA mobile communication system that comprises a base station controller, a plurality of mobile stations and base stations. The base stations are discriminated using different sequences. The method includes: (a) assigning a group code of the cell as a pilot code of an inphase channel of the base station; (b) assigning a cell code of the cell as a pilot code of a quadrature channel of the base station; and (c) multiplexing the pilot codes of the inphase channel and the quadrature channel, and generating an inphase/quadrature pilot code.
Referring to FIG. 1, a schematic view illustrates the frame structure of a 3GPP wide-band CDMA system. In a 3GPP wide-band CDMA system, the cell search is typically performed through three stages that include two specifically designed synchronization channels (SCH) and one common pilot channel (CPICH). In a first stage 110, a primary synchronization channel (PSCH) 111 is used for slot synchronization. The PSCH 111 includes a primary synchronization code (PSC) referred to as acp, wherein “a” (=±1) depends on the existence of a diversity transmission at the base station. In the second stage 120, the frame/code group is identified by means .of a secondary synchronization channel (SSCH) 121. The secondary synchronization channel 121 includes a secondary synchronization code (SSC) referred to as acs, wherein the coefficient “a” is equal to that of the primary synchronization code. In the third stage 130, the downlink scrambling code is determined by means of a common pilot channel 131. As illustrated, a 10 ms-long frame includes 15 slots. Because the system has a chip rate of 3.84 Mchips/sec, each frame therefore includes 38400 chips, and each slot includes 2560 chips. Furthermore, the primary synchronization channel and the secondary synchronization channel are 256 chips-long and only transmit at the beginning of the slot boundary.
The cell search conventionally implemented in the 3GPP wide-band CDMA/FDD system can be usually divided into two general types: the serial cell search method and the pipelined cell search. Before a new cell search trial is started, the serial cell search needs to undergo three successive synchronization stages. These three synchronization stages include: (1) slot synchronization; (2) frame synchronization/code group identification; and (3) scrambling code identification.
Referring to FIG. 2, a schematic diagram illustrates a serial search method used in a 3GPP W-CDMA system in the prior art, in which the processing time of each stage being 10 ms. The completion of a sequence of the three stages is usually called a trial, while a successful cell search comes after the first successful trial. In a serial cell search, the trials do not overlap one another. In other words, only one single stage is conducted at a time, i.e. only one block 211 (or block 212 or block 213) is conducted at a time (each block represents a stage of one trial). Less power is therefore consumed, but the cell search time is negatively longer.
Referring to FIG 3, a simplified schematic diagram illustrates the pipelined cell search method known in the prior art, in which the three stages of different trials are concurrently conducted (i.e. three-stage cell search are operated in parallel). In other words, the different trials overlap one another. For example, the blocks 311, 321, and 331 belong to the same trial. In the pipelined cell search method, more trials are allowed within a given time interval. Therefore, a faster cell search may be accomplished. However, this method consumes more power. For example, if each stage requires 10 ms, and the pipelined cell search is successful at the K-th trial, the cell search needs a cell search time of (K+2)×10 (ms). In contrast, a serial cell search would need K×30 (ms). Notice that compared to the serial cell search method, the implementation of the pipelined cell search method does not require additional hardware.
However, the implementation of the cell search methods of the prior art in a 3GPP W-CDMA/FDD system requires the assumption of two premises. First, the outputted sample from the chip-matched filter is ideal. Secondly, the chip clock of the transmitter is precisely known to the receiver (i.e. no clock offset occurs). In other words, the chip duration of the incoming signal is not subject to any frequency offset. Practically, the above assumptions are rarely met. Non-ideal sampling effects usually occur due to the uncertain propagation delay, and the frequency offset is most of the time caused by frequency instability of the crystal oscillator in the user equipment. The frequency offset in the digital receiver produces two effects: (1) a phase offset; and (2) a clock offset. Unfortunately, the clock offset generally is not considered in the prior art. This clock offset, caused by the frequency offset, however exists between the base station and the user equipment. As illustrated in Table.1, different conditions of frequency offset produce corresponding clock offset effects. For example, if a frequency offset of 12 kHz occurs, a frame of 30 ms is likely to include an offset of about 0.69 times the chip length, which is approximately a clock offset of 6 ppm. This would result in a signal difference and increase the cell search time.
FIG. 4(a) and FIG. 4(b) are schematic graphs illustrating the resulting signal level and inter-chip interference under the presence of the clock offset, caused by a frequency offset. As illustrated, due to the clock offset, the signal level is deteriorated and the inter-chip interference increases.